Linear solenoid locks and brakes have many applications. As an example, such locks and brakes find application in aircraft where safety is critical and where it is necessary to lock/maintain components such as flaps, doors, brakes etc. in position.
FIG. 1 is a schematic view of a solenoid lock comprising an armature or plunger, held under a spring force and linearly moveably arranged within a solenoid coil. When the coil is de-energized, the spring bias will keep the plunger in a first state. When the coil is energized, the resulting magnetic force will overcome the spring force and move the plunger to a second state or linear position. Depending on the application the first state can be a locked state and the second state an unlocked state or vice versa.
Voltage and temperature variations, for example, can affect the position of the plunger using the power on/power off control. To address this, and improve reliability and, hence, safety, a two-level closed-loop control has been used. Here, as seen in FIG. 2, to move the plunger to the second state (can be open or locked depending on application) a first, high level of current (pull-in current) is applied to the solenoid for a predefined period of time (usually longer than its response time in moving to the second state), after which a lower level of current (holding current) is applied to maintain the position. This reduces power losses and temperature rises compared to constantly applying the higher pull-in current.
Although the time period during which the higher pull-in current is supplied is selected to be more than long enough, in normal circumstances, to linearly move the plunger to its end position for the second state, situations can arise that prevent the plunger reaching the end position before the pull-in current is stopped. Mechanical or electrical abnormalities or disturbances can cause the response time to be longer than expected, or mechanical jams can occur. This gives rise to safety issues, e.g. trying to move components while a brake/lock is still locked or the component not being fully locked or arrested.
To overcome these problems, systems have been developed that provide position feedback, rather than relying on an expected response time period. Such systems use a sensor such as a proximity sensor to detect the position of the plunger relative to its end position and to feed this back to the system controller. Such sensors, however, considerably add to the complexity, cost, weight, size and wiring of the system, and more so where dissimilarity (i.e. the use of different types of sensors to prevent common mode failure) or redundancy is required (i.e. two or more sensors) for safety-critical applications.
The present invention aims to provide improved monitoring of a plunger position of a solenoid.